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See also: All About EASE-Grid
The figure above is an example of the EASE-Grid time file and gridded SSM/I Brightness temperatures for one channel on December 31, 1989. Data in both figures represent ascending passes only. The data in the upper figure represent the time (GMT) of the sample from which the corresponding interpolated Brightness Temperature data (19 GHz, horizontal polarization channel) in the lower figure were derived.
At latitudes of approximately 55 degrees and poleward, there are multiple orbital passes over the same geographic area during a single day. Where these multiple passes occur, the pass whose local time is closest to the equator crossing time (ascending or descending) is selected. This is done in order to provide a consistent set of sample times for each daily image file. Within a given daily image (ascending or descending) the local times would not be expected to differ by more than about 100 minutes (the orbital period). This method results in a systematic spatial progression through GMT as seen in the "spiral" pattern of the upper figure.
Please refer to EASE-Grid, A Versatile Set of Equal-Area Projections and Grids for more information on the complete data set.